What can I use instead of fish sauce for guests who are allergic?

Usually an allergy to fish means allergy to: all shellfish, seaweed (which contains a lot of iodine), also some white fish and even salmon. The allergy is an allergy to iodine found in seafood and seaweed etc. so, don’t risk it by substituting anything containing anything from the sea. I have a severe allergy to shellfish and cannot even eat at a restaurant where they are steaming lots of shellfish. It is concentrated in the air when it is cooking. Darn it, wish I could eat it!

Your local health food store (or maybe even whole foods) will have Vegan Fish sauce. Give them a call..

Here's a copy cat recipe.
http://www.thekitchn.com/recipe-vegan-fish-sauce-130535

The base flavor is wakame seaweed, with additions of 'mushroom' soy sauce, whatever that is, garlic, and miso.

Use white soy sauce, also sometimes referred to as "thin" soy sauce. This may be what Kristen W. is referring to. It is a very good replacement for fish sauce; of course not the same, but much, much better than using dark soy sauce, especially in an otherwise lightly flavored dish or in a dipping sauce.

It depends on what you're making, and also what in fish sauce causes the reaction. If they're only allergic to specific fish (e.g., anchovies,) but not all seafood, perhaps you could use tiny Asian dried shrimp - but definitely find out if shrimp is a problem, as it's a common allergy too.

To the above I would add that, in my experience, Vietnamese soy sauce is a bit lighter and funkier than Japanese or Chinese soy sauce and while it still won't replicate the taste of fish sauce, it's flavor is a bit more "in the family" of the taste of fish sauce than those other kinds. I would recommend trying that if you have a Vietnamese or Southeast Asian market near you.

I agree with Susan W - it's almost impossible to replicate. But if you take into account what they're allergic to (is it shellfish or the anchovy in most fish sauce), what you want to replicate (salty, fermented, umami) and the cuisine of the recipe (is it Asian, which? or western using fish sauce) you can decide what to increase or substitute. So yes soy or mushroom sauce, also oyster sauce, any alcoholic drink that fits the cuisine or recipe. Good luck...

It depends what you are making, but there really is no substitute. I would just up the amount of whatever other umami ingredients are in the recipe. Soy sauce, shiitake mushrooms etc. Some people who do not have access to fish sauce will use an anchovy, but that must be what they are used to?